1. Technical Field
The present invention relates to a pressure sensor, in particular, a pressure sensor that does not use oil as a pressure receiving medium.
2. Related Art
There have been known pressure sensors using a piezoelectric resonator as a pressure sensitive element, such as a water pressure gauge, an air gauge, and a differential pressure gauge. For example, in a piezoelectric resonator, an electrode pattern is formed on a planar piezoelectric substrate, and a detecting axis is set in the direction of detecting a force. When pressure is applied to the piezoelectric resonator in the direction of the detecting axis, the resonance frequency of the piezoelectric resonator varies. The pressure is detected on the basis of the variation in the resonance frequency. JP-A-56-119519, JP-A-64-9331, and JP-A-2-228534 disclose pressure sensors using a piezoelectric resonator as a pressure sensitive element. When pressure is applied to a bellows via a pressure input orifice, a force F according to the effective area of the bellows is applied to the piezoelectric resonator as a compressive force or a tensile force via a force transmitting unit using a pivot (a flexible hinge) as a fulcrum. Stress corresponding to the force F occurs in the piezoelectric resonator. The resonance frequency of the piezoelectric resonator varies due to this stress. The pressure sensor measures the pressure by detecting the variation in the resonance frequency of the piezoelectric resonator.
Hereafter, a related-art pressure sensor will be described using examples disclosed in JP-A-56-119519 and the like. FIG. 13 is a schematic view showing a structure of a related-art pressure sensor.
A related-art pressure sensor 501 shown in FIG. 13 includes a case 504 having a first pressure input orifice 502 and a second pressure input orifice 503 made in an opposed manner and a force transmitting member 505 disposed inside the case 104. One ends of a first bellows 506 and a second bellows 507 are connected to one end of the force transmitting member 505 in such a manner that the one end of the force transmitting member 505 is interposed between these bellows. The other end of the first bellows 506 is coupled to the first pressure input orifice 502, and the other end of the second bellows 507 is coupled to the second pressure input orifice 503. Also, a double-ended tuning fork resonator 509 serving as a pressure sensitive element is disposed between the other end of the force transmitting member 505 and an end of a substrate 508 opposite to an end thereof serving as a pivot (fulcrum).
In general, a bellows of a pressure sensor is filled with a liquid so that pressure is detected with high accuracy. As such a liquid, oils having high viscosity, such as a silicon oil, are typically used in order to prevent bubbles from entering the bellows or staying in the folds thereof.
For this reason, the first bellows 506 is filled with an oil 510 having viscosity. If a liquid is the subject of pressure measurement, the liquid and the oil 510 make contact with and face each other via an opening 511 made at the first pressure input orifice 502. The diameter of the opening 511 is set to a size such that the oil 510 does not leak out.
In the pressure sensor 501 having the above-mentioned structure, when the pressure F is applied to the oil 510 filling the first bellows 506 by the liquid, which is the subject of pressure measurement, the pressure F is applied to the one end of the force transmitting member 505 (a swing lever supported by a pivot) via the first bellows 506. On the other hand, atmospheric pressure is applied to the second bellows 507. Thus, a force equivalent to the atmospheric pressure is applied to the one end of the force transmitting member 505.
As a result, a force that is equivalent to the differential pressure between the pressure F applied by the liquid, which is the subject of pressure measurement, and pressure based on the atmospheric pressure is applied to the double-ended tuning fork resonator 509 as a compressive force or a tensile force via the other end of the force transmitting member 505 using the pivot of the substrate 508 as a fulcrum. Thus, stress occurs in the resonator 509 and the resonance frequency of the resonator 509 varies with the magnitude of the stress. Therefore, by measuring the resonance frequency, the magnitude of the pressure F is detected.
JP-A-2005-121628 presents a pressure sensor that does not include a costly force transmitting unit (cantilever) having a swing lever using a pivot (flexible hinge) as a fulcrum, as used in the above-mentioned pressure sensor. In this pressure sensor, two bellows are aligned with a pedestal interposed therebetween in a housing. The pressure sensor is intended to detect a pressure variation attributable to the difference between pressures applied to the bellows on the basis of a movement of a pedestal along the direction of expansion or contraction of the bellows. Specifically, a resonator bonding pedestal is interposed between one end of a first bellows and one end of a second bellows. Both ends of a pressure sensitive element are fixed to the pedestal and a housing wall adjacent to the other end of the second bellows on the circumference of the second bellows. A reinforcing plate is disposed in such a manner that the reinforcing plate is symmetrical with the pressure sensitive element with respect to the second bellows. Both ends of the reinforcing plate are fixed to the pedestal and the housing wall.
In order to solve the problem that the bellows of the pressure sensor disclosed in JP-A-2005-121628 do not have sufficient strength against shock applied from a direction orthogonal to the direction of a pressure detecting axis, JP-A-2007-57395 proposes a pressure sensor where a pedestal and a housing are connected in a direction orthogonal to the direction of a pressure detecting axis using a reinforcing flexible member (that is, a spring).
JP-A-2006-194736 discloses a pressure sensor that is intended to detect hydraulic pressure inside an engine and is used in such a manner that it is fixed to an engine block. This pressure sensor includes a sensing unit for outputting an electrical signal according to an applied pressure, a pressure receiving diaphragm for receiving pressure, and a pressure transmitting member for transmitting the pressure from the diaphragm to the sensing unit. Specifically, a first diaphragm for receiving pressure is provided on one end surface of a hollow metal stem and a second diaphragm for detection is provided on the other end surface thereof. The pressure transmitting member is provided between the first and second diaphragms in the stem. The pressure transmitting member is a shaft made of metal or ceramic and is provided between the pair of diaphragms in a prestressed state. A chip with a strain gauge function of a pressure detection element is stuck on the outer end surface of the second diaphragm. Pressure received by the first diaphragm is transmitted to the second diaphragm by the pressure transmitting member so that the second diaphragm becomes deformed. The deformation of the second diaphragm is converted into an electrical signal by the strain gauge chip. Thus, the hydraulic pressure of the engine is detected.
JP-A-2007-132697 discloses a pressure sensor that includes: a pressure receiving diaphragm that has one surface serving as a pressure receiving surface on which pressure to be measured acts and that becomes distorted by receiving the pressure to be measured; a pressure transmitting member having one end in contact with the other surface of the pressure receiving diaphragm; a metal stem distortion portion to which the pressure to be measured transmitted from the pressure receiving diaphragm is applied via the pressure transmitting member; and a sensing unit that is a semiconductor chip for generating a signal on the basis of a distortion of the distortion portion. The pressure receiving diaphragm takes a waveform that is folded multiple times. In the pressure receiving diaphragm, the central area in contact with the pressure transmitting unit is thick and the area surrounding the thick central area takes a waveform. Thus, the mechanical strength of the diaphragm is improved while the spring characteristic thereof is ensured. The diaphragm is formed by means of stamping or cutting.
JP-A-08-159900 and JP-A-07-19981 propose specific configurations of pressure receiving diaphragms for use in a pressure sensor.
JP-A-08-159900 describes a diaphragm having recesses and protrusions in section. A thin, flat portion is provided in the center of the pressure receiving portion of the diaphragm. As shown in FIG. 1 of JP-A-08-159900, the thick peripheral area of the diaphragm restrains the influence on the central area, of a distortion caused when welding the diaphragm to the housing. Recesses and protrusions are made on the diaphragm by half-etching both surfaces of a thin metal plate.
JP-A-07-19981 describes a pressure-resistant sensor for high temperature where a pressure transmitting member is provided between the central area of a pressure receiving diaphragm for receiving pressure to be measured and a distortion sensitive element and the pressure is detected by transmitting a distortion of the pressure receiving diaphragm to the distortion sensitive element. In order to prevent the diaphragm from expanding and thus becoming deformed when the diaphragm is subjected to high temperature in an engine and thus causing an error in sensing, the central area that is originally thicker than the peripheral area of the diaphragm is recessed so that an output error attributable to a variation in the thermal distortion amount of the diaphragm caused by a variation in the temperature difference in the thickness direction of the diaphragm is reduced.
However, in JP-A-56-119519, JP-A-64-9331, and JP-A-2-228534, as shown by the pressure sensor 501 of FIG. 13, the oil 510 filling the first bellows first bellows 506 has a larger thermal expansion coefficient than those of other elements constituting the pressure sensor 501, such as the force transmitting member 505 and the double-ended tuning fork resonator 509. Therefore, when the temperature changes, thermal distortions occur in the elements constituting the pressure sensor 501. Such thermal distortions act on the double-ended tuning fork resonator 509 as unwanted stress, resulting in an error of a measured pressure value. Thus, the characteristics of the pressure sensor are disadvantageously degraded.
Also, the oil 510 filling the first bellows 506 makes contact with and faces a liquid, which is the subject of pressure measurement. Therefore, depending on how the pressure sensor is installed, the oil 510 may flow out toward the liquid or the liquid may flow into the first bellows 506. In this case, bubbles may occur in the oil 510. If bubbles occur in the oil 510, the oil 510 serving as a pressure transmitting medium can no longer stably transmit a force to the double-ended tuning fork resonator 509 via the force transmitting member 505. Thus, an error may occur in a measured pressure value.
Also, as described above, the oil 510 makes contact with and faces the liquid, which is the subject of pressure measurement; therefore, depending on how the pressure sensor is installed, the oil 510 may flow out toward the liquid. Therefore, disadvantageously, the related-art pressure sensor using the oil 510 cannot be used to measure the pressure of a pure liquid into which no foreign substance must be mixed.
Also, the related-art pressure sensor 501 includes the force transmitting member 505 having a complicated structure. This is an obstacle to downsizing the pressure sensor. Also, the force transmitting member 505 requires a flexible hinge having a slim constriction. This disadvantageously makes the force transmitting unit 505 a costly component, thereby increasing the manufacturing cost of the pressure sensor.
When the pressure sensors proposed by JP-A-2005-121628 and JP-A-2007-57395 are inclined, the bellows droop. Thus, a force applied to the pressure sensitive element such as a double-ended tuning fork resonator varies, resulting in a variation in the resonance frequency.
Also, in the pressure sensors proposed by JP-A-2005-121628 and JP-A-2007-57395, one end of a pipe filled with an oil is connected to a pressure introduction orifice and the other end of the pipe is brought into contact with a liquid to be measured. Therefore, as is the case with JP-A-56-119519, JP-A-64-9331, and JP-A-02-228534, the oil filling the bellows and the pipe makes contact with and faces the liquid. Accordingly, depending on how the pressure sensors are installed, the oil may flow out toward the liquid or the liquid may flow into the bellows. In this case, bubbles may occur in the oil. If bubbles occur in the oil, the oil serving as a pressure transmitting medium can no longer stably transmit a force to the double-ended tuning fork resonator via the pedestal. This disadvantageously results in an error of the measured pressure value.
As for JP-A-2007-57395, the pedestal interposed between the bellows is supported on the lateral surface of the housing by the reinforcing flexible member, which is a plate spring. For this reason, when the bellows move in the axis direction, a force restraining a movement of the pedestal works. As a result, the pressure detecting sensitivity may be deteriorated. Also, if the hardness of the reinforcing flexible member is increased so that the reinforcing flexible member supports the pedestal more firmly, movements of the bellows are restrained. Thus, the pressure detecting sensitivity is disadvantageously deteriorated.
Also, in JP-A-2005-121628 and JP-A-2007-57395, the reinforcing plate is disposed in such a manner that it is symmetrical with the pressure sensitive element with respect to the bellows, so movements of the bellows are restrained. This disadvantageously deteriorates the pressure detecting sensitivity.
In JP-A-2006-194736 and JP-A-2007-132697, the prestressed diaphragm and center shaft are in contact with each other. If the diaphragm and center shaft are rigidly fixed in the pressure sensor that is used under high temperature and high pressure, the mechanism may be broken due to the difference in thermal expansion between these elements. Therefore, in consideration of such thermal expansion, the diaphragm and center shaft have only a point contact with each other and are not bonded together using an adhesive or the like. Therefore, there is a very high possibility that when the diaphragm and center shaft operate due to a variation in the pressure, the contact point is misaligned. If the contact point is misaligned, a force acting on both the diaphragm and center shaft leaks out. This disadvantageously prevents the pressure sensor from detecting the pressure with high accuracy. Also, the pressure sensors described in JP-A-2006-194736 and JP-A-2007-132697 are originally used under high temperature and high pressure. For this reason, it is desirable that the force transmitting member be as long as possible so as to secure a distance between the pressure receiving portion and the sensing unit so that thermal effect on such as a chip of the sensing unit is avoided. Therefore, it is not preferable to apply these pressure sensors to technologies for downsizing. Also, in JP-A-2006-194736 and JP-A-2007-132697, the center shaft is provided between the pair of diaphragms so that a force is transmitted, and the sensor chip is attached to the diaphragm in the sensing unit. For this reason, the properties of the portions adjacent to the pressure receiving portion, of the diaphragms are different from those of the portions adjacent to the sensing unit, thereof. This is a major disadvantage in that the measuring accuracy cannot be increased.
As for the diaphragm described in JP-A-08-159900, the central area of the pressure receiving surface is thin; therefore, when an external force such as pressure is applied to the central area, the central area is easily damaged. For this reason, the diaphragms having a thick central area, described in JP-A-2007-132697 and JP-A-07-19981, are considered. The related-art example described in JP-A-2007-132697 does not have the problems that the above-mentioned JP-A-56-119519, JP-A-64-9331, and JP-A-02-228534 have; however, it has, for example, a problem that the material of the diaphragm is limited only to metal such as stainless steel and a problem that such a material is not suited to downsizing, since the diaphragm is formed by means of stamping. Also, the waveform diaphragm formed by means of stamping, disclosed in JP-A-2007-132697, disadvantageously requires a process of eliminating residual stress applied to the diaphragm during stamping, as a post process. Further, if, in JP-A-2007-132697, an attempt is made to manufacture a diaphragm using the photolithography technique and etching technique (hereafter collectively referred to as “photolitho-etching”) described in JP-A-08-159900, the front surface and back surface of the diaphragm are apt to be misaligned and therefore it is difficult to perform photolitho-etching with high yield, since both the diaphragms described in JP-A-2007-132697 and JP-A-08-159900 have a complicated structure. Also, the force transmitting unit according to JP-A-2007-132697 is point-connected to the diaphragm, so stress is concentrated on the portion where the pressure transmitting member and diaphragm are connected. Thus, aged deterioration is disadvantageously apt to occur.
As for JP-A-07-19981, the structure of the diaphragm shown in FIG. 1 thereof has the above-mentioned, stamping-related problem, as in JP-A-2007-132697. Also, the structures shown in FIGS. 2 to 6 thereof have a problem that a protection member serving as a heat shield plate must be provided on the pressure receiving surface of the diaphragm and thus the manufacturing process is complicated.